Bandpass filter is a necessary component for wireless communication products, and its main function is to separate frequencies. In other words, the bandpass filter can let a signal within a certain specific frequency band pass and block any signal other than those within the specific frequency band. As the wireless communication market booms, the requirement of communication quality becomes increasingly higher, and signal receivers require a wideband and high-efficiency bandpass filter to process the received high-frequency signal. The high-efficiency bandpass filter not only filters unnecessary interference signals, but also provides a wideband utility rate and good receiving efficiency to the high frequency signals.
As the bandwidth becomes wider and wider, data transfer rate increases significantly, so that researches and applications related to wireless transmission at a band near 60 GHz become more and more important in recent years. With a standard established by the Federal Communications Commission (FCC), any wireless communication at a band near 60 GHz (i.e. 57˜64 GHz) enjoys the right of using free bandwidth about 7 GHz, and thus a wireless HD group formed by international major communication companies including LG, Panasonic, NEC, Samsung, Sony and Toshiba promotes that high resolution video without compression but with resolution up to 1920×1080 p can be wirelessly transmitted at the band of 60 GHz. In high frequency transmission, the band of 60 GHz can thoroughly implement wireless communication and high speed transmission in our daily life.
Various bandpass filters used in conventional commercial Wi-Fi and Bluetooth products are available in the market, and the most popular one among these products is the transmission-line bandpass filter, whose single-layer or double-layer metal structure can be integrated with other components directly on a printed circuit board. For example, Hong-Hong Hu et al. proposed “Novel Compact Ultra-Wideband Filter with Wide Stop-Band” in Microwave and Optical Technology Letters, Vol. 51, No. 1, pp. 53-55, January 2009, which discloses a transmission-line bandpass filter for general Wi-Fi band, and uses a signal transmission head incorporated with two T-shaped resonators to obtain a bandpass filtering effect. However, so far, the present 60 GHz filters still have the issues of high loss, a lower selectivity factor between a passband and a stopband, and limited extension of the stopband. Furthermore, group hysteresis of the transmission-line bandpass filter may cause signal waveform distortion. Besides, the conventional bandpass filter needs more elements and leads to higher design complexity and higher cost. Therefore, a simple-structure and low-cost bandpass filter is extremely desired to be developed.